While Nd—Fe—B sintered magnets, referred to as Nd magnets, hereinafter, are regarded as the functional material necessary for energy saving and performance improvement, their application range and production volume are expanding every year. Since many applications are used in high temperature, the Nd magnets are required to have not only a high remanence but also a high coercivity. On the other hand, since the coercivity of Nd magnets are easy to decrease significantly at a elevated temperature, the coercivity at room temperature must be increased enough to maintain a certain coercivity at a working temperature.
As the means for increasing the coercivity of Nd magnets, it is effective to substitute Dy or Tb for part of Nd in Nd2Fe14B compound as main phase. For these elements, there are short resource reserves in the world, the commercial mining areas in operation are limited, and geopolitical risks are involved. These factors indicate the risk that the price is unstable or largely fluctuates. Under the circumstances, the development for a new process and a new composition of R—Fe—B magnets with a high corecivity, which include a minimizing the content of Dy and Tb, is required.
From this standpoint, several methods are already proposed. Patent Document 1 discloses an R—Fe—B base sintered magnet having a composition of 12-17 at % of R (wherein R stands for at least two of yttrium and rare earth elements and essentially contains Nd and Pr), 0.1-3 at % of Si, 5-5.9 at % of B, 0-10 at % of Co, and the balance of Fe (with the proviso that up to 3 at % of Fe may be substituted by at least one element selected from among Al, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, In, Sn, Sb, Hf, Ta, W, Pt, Au, Hg, Pb, and Bi), containing a R2(Fe,(Co),Si)14B intermetallic compound as main phase, and exhibiting a coercivity of at least 10 kOe. Further, the magnet is free of a B-rich phase and contains at least 1 vol % based on the entire magnet of an R—Fe(Co)—Si phase consisting essentially of 25-35 at % of R, 2-8 at % of Si, up to 8 at % of Co, and the balance of Fe. During sintering or post-sintering heat treatment, the sintered magnet is cooled at a rate of 0.1 to 5° C./min at least in a temperature range from 700° C. to 500° C., or cooled in multiple stages including holding at a certain temperature for at least 30 minutes on the way of cooling, for thereby generating the R—Fe(Co)—Si phase in grain boundary.
Patent Document 2 discloses a Nd—Fe—B alloy with a low boron content, a sintered magnet prepared by the alloys, and their process. In the sintering process, the magnet is quenched after sintering below 300° C., and an average cooling rate down to 800° C. is ΔT1/Δμl<5K/min.
Patent Document 3 discloses an R-T-B magnet comprising R2Fe14B main phase and some grain boundary phases. One of grain boundary phase is R-rich phase with more R than the main phase and another is Transition Metal-rich phase with a lower rare earth and a higher transition metal concentration than that of main phase. The R-T-B rare earth sintered magnet is prepared by sintering at 800 to 1,200° C. and heat-treating at 400 to 800° C.
Patent Document 4 discloses an R-T-B rare earth sintered magnet comprising a grain boundary phase containing an R-rich phase having a total atomic concentration of rare earth elements of at least 70 at % and a ferromagnetic transition metal-rich phase having a total atomic concentration of rare earth elements of 25 to 35 at %, wherein an area proportion of the transition metal-rich phase is at least 40% of the grain boundary phase. The green body of magnet alloy powders is sintered at 800 to 1,200° C., and then heat-treated with multiple steps. First heat-treatment is in the range of 650 to 900° C., then sintered magnet is cooled down to 200° C. or below, and second heat-treatment is in range of at 450 to 600° C.
Patent Document 5 discloses an R-T-B rare earth sintered magnet comprising a main phase of R2Fe14B and a grain boundary phase containing more R than that of the main phase, wherein easy axis of magnetization of R2Fe14B compound is in parallel to the c-axis, the shape of the crystal grain of R2Fe14B phase is elliptical shape elongated in a perpendicular direction to the c-axis, and the grain boundary phase contains an R-rich phase having a total atomic concentration of rare earth elements of at least 70 at % and a transition metal-rich phase having a total atomic concentration of rare earth elements of 25 to 35 at %. It is also described that magnet are sintered at 800 to 1,200° C. and subsequent heat treatment at 400 to 800° C. in an argon atmosphere.
Patent Document 6 discloses a rare earth magnet comprising R2T14B main phase and an intergranular grain boundary phase, wherein the intergranular grain boundary phase has a thickness of 5 nm to 500 nm and the magnetism of the phase is not ferromagnetism. It is described that the intergranular grain boundary phase is formed from a non-ferromagnetic compound due to add element M such as Al, Ge, Si, Sn or Ga, though this phase contains the transition metal elements. Furthermore by adding Cu to the magnet, a crystalline phase with a La6Co11Ga3-type crystal structure can be uniformly and widely formed as the intergranular grain boundary phase, and a thin R—Cu layer may be formed at the interface between the La6Co11Ga3-type grain boundary phase and the R2T14B main phase crystal grains. As a result, the interface of the main phase is passivated, a lattice distortion of main phase can be suppressed, and nucleation of the magnetic reversal domain can be inhibited. The method of preparing the magnet involves post-sintering heat treatment at a temperature in the range of 500 to 900° C., and cooling at the rate of least 100° C./min, especially at least 300° C./min.
Patent Document 7 and 8 disclose an R-T-B sintered magnet comprising a main phase of Nd2Fe14B compound, an intergranular grain boundary which is enclosed between two main phase grains and which has a thickness of 5 nm to 30 nm, and a grain boundary triple junction which is the phase surrounded by three or more main phase grains.